Foam causes serious operating problems in pulp and paper mills. It is particularly troublesome in the paper machine white water stage of the operation. Lack of adequate foam control in the pulp slurry may result in the curtailment of production or diminished product quality.
Foam is a colloidal system in which a gas is dispersed in a liquid. Foam can either exist as bubbles of entrained air in the bulk medium or as a combination of entrained gases and surface foam. Foams are thermodynamically unstable but are stabilized by two mechanisms:
1. The adsorption at the air/water interface of surface active materials such as salts of rosin acids, fatty acids and lignin. PA0 2. The concentration of finely divided solid particles around each bubble such as starch, cellulose, fines, fillers, etc. PA0 (c) film rupture
Foam in residual black pulping liquor is believed to be stabilized primarily by the first mechanism; whereas foam in paper machine white water is stabilized primarily by the second mechanism.
Defoamers, which, when added to a foaming liquid, prevent the formation of bubbles or cause small bubbles to coalesce are well known to those skilled in the art. Two different theories, dewetting and surfactant adsorption, have been used to explain lamella rupture by these hydrophobic materials. In the dewetting mechanism, the hydrophobic particle enters the air-liquid surface in the lamella. When the lamella drains sufficiently, due to gravitational and capillary forces, the particle dewets through both sides of the lamella which causes a hole in the film and in turn initiates rupture. The contact angle between the film and the particle as well as the particle size and shape are very important in the dewetting mechanism.
In the surfactant adsorption mechanism, the hydrophobic particle enters the aqueous phase through the lamella surface; it then adsorbs a layer of surfactant which leaves an area of the surface depleted of surfactant and is rendered susceptible to rupture due to mechanical, gravitational or capillary forces. This mechanism is debatable because it is known that the lamella can "heal" quickly via the Marangoni Effect. Thus the dewetting mechanism is believed to be more generally applicable.
Most conventional defoamers comprise a hydrophobic material having a melting point greater than 40.degree. C. or an insoluble material such as silica, dispersed in an oil phase. The production of these compositions requires the separate steps of heating and cooling. Due to the use of high melting point hydrophobic materials, the resulting composition quickly becomes unstable and the propensity for deposition in the treated aqueous system is very high.
It is one object of the present invention to provide an improved foam control composition and method of using same in pulp and paper processing. It is a further object to obviate the necessity of having to process the composition at elevated temperatures. Still further objects are to produce a foam control composition that remains stable before use and which will reduce deposition potential in the treated system.